I think people would be shocked to find out just how common "don't ask, don't tell" is among couples, especially among the rich, famous, and powerful. Everyone ganged up on Jada Smith until Will admitted he too sought outside "entanglements." Most couples who do this just hide it well enough, and it's only a problem when one of them is caught.

Not all, very likely not even most, of course, do this, and it's not always truly mutual or healthy, but surprise surprise, not every couple wants strict monogamy. They just don't want society to judge them for it.